![Rosetta_triumphs_at_asteroid_Lutetia](https://upload.wikimedia.org/wikipedia/commons/1/17/Rosetta_triumphs_at_asteroid_Lutetia.jpg)
M-type (aka M-class) asteroids are a
spectral class
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
of
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
s which appear to contain higher concentrations of metal phases (e.g. iron-nickel) than other asteroid classes,
and are widely thought to be the source of
iron meteorite
Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most iron met ...
s.
Definition
Asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
s are classified as M-type based upon their generally featureless and flat to red-sloped absorption
spectra in the visible to near-infrared and their moderate optical
albedo
Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
. Along with the spectrally similar
E-type and
P-type asteroids (both categories E and P were formerly type-M in older systems), they are included in the larger
X-type asteroid group and are distinguishable only by optical albedo:
:
Characteristics
Composition
Although widely assumed to be metal-rich (the reason for use of "M" in the classification), the evidence for a high metal content in the M-type asteroids is only indirect, though highly plausible. Their spectra are similar to those of
iron meteorite
Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most iron met ...
s and
enstatite chondrite
Enstatite chondrites (E-type chondrites) are a rare form of meteorite, rich in the mineral enstatite. Only about 200 E-Type chondrites are currently known, comprising about 2% of the chondrites that fall on Earth.Norton, O.R. and Chitwood, L.A. F ...
s,
and radar observations have shown that their
radar albedos are much higher than other asteroid classes,
consistent with the presence of higher density compositions like iron-nickel.
Nearly all of the M-types have radar albedos at least twice as high as the more common
S- and
C-type, and roughly one-third have radar albedos ~3× higher.
High resolution spectra of the M-type have sometimes shown subtle features longward of 0.75
μm and shortward of 0.55 μm.
The presence of silicates is evident in many,
and a significant fraction show evidence of absorption features at 3 μm, attributed to hydrated silicates.
The presence of silicates, and especially hydrated silicates, is at odds with the traditional interpretation of M-types as remnant iron cores.
Bulk density and porosity
The
bulk density Bulk density, also called apparent density or volumetric density, is a property of powders, granules, and other "divided" solids, especially used in reference to mineral components (soil, gravel), chemical substances, (pharmaceutical) ingredients ...
of an asteroid provides clues about its composition and meteoritic analogs.
For the M-types, the proposed analogs have bulk densities that range from ~3 for some types of
carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small prop ...
s up to nearly 8 for the iron-nickel present in
iron-meteorites.
Given the bulk density of an asteroid and the density of the materials that make it up (aka particle or grain density), one can calculate its
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
and infer something of its internal structure; for example, whether an object is coherent, a
rubble pile
In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the ...
, or something in-between.
To calculate the bulk density of an asteroid requires an accurate estimate of its mass and volume; both of these are difficult to obtain given their small size relative to other solar system objects. In the case of the larger asteroids, one can estimate mass by observing how their gravitational field affects other objects, including other asteroids and orbiting or flyby spacecraft.
If an asteroid possesses one or more
moons
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
, one can use their collective orbital parameters (e.g. orbital period, semimajor axis) to estimate the masses of the ensemble, for example in the
two-body problem
In classical mechanics, the two-body problem is to predict the motion of two massive objects which are abstractly viewed as point particles. The problem assumes that the two objects interact only with one another; the only force affecting each ...
.
To estimate an asteroid's volume requires, at a minimum, an estimate of an asteroid's diameter. In most cases, these are estimated from the
visual albedo (brightness) of the asteroid, chord-lengths during
occultations
An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...
, or their thermal emissions (e.g.
IRAS mission). In a few cases, astronomers have managed to develop three-dimensional shape models using a variety of techniques (c.f.
16 Psyche
16 Psyche () is a large M-type asteroid discovered by the Italian astronomer Annibale de Gasparis on 17 March 1852 and named after the Greek goddess Psyche.
The prefix "16" signifies that it was the sixteenth minor planet in order of dis ...
or
216 Kleopatra for examples) or, in a few lucky instances, from spacecraft imaging (c.f
162173 Ryugu
162173 Ryugu, provisional designation , is a near-Earth object and a potentially hazardous asteroid of the Apollo group. It measures approximately in diameter and is a dark object of the rare spectral type Cb, with qualities of both a C-type ...
).
Of these, mass measurements made via spacecraft deflection or the orbits of moons are considered the most reliable. Ephemeris estimates are based on the subtle gravitational pull of other objects on that asteroid, or vice versa, and are considered less reliable. The exception to this caveat may be Psyche, as it is the most massive M-type asteroid and has numerous mass estimates.
Size estimates based on shape models (usually derived from adaptive optics, occultations, and radar imaging) are the most reliable. Direct spacecraft imaging (Lutetia) is also quite reliable. Sizes based on indirect methods like thermal IR (e.g. IRAS) and radar echoes are less reliable.
None of the M-type asteroids have bulk densities consistent with a pure iron-nickel core. If these objects are porous (aka
rubble-piles), then that interpretation may still hold; however, the consensus is that this is unlikely for Psyche,
because of its large size. Given the spectral evidence of silicates on most M-type asteroids, the consensus interpretation for most of these larger asteroids is that they are composed of lower density meteorite analogs (e.g.
enstatite chondrite
Enstatite chondrites (E-type chondrites) are a rare form of meteorite, rich in the mineral enstatite. Only about 200 E-Type chondrites are currently known, comprising about 2% of the chondrites that fall on Earth.Norton, O.R. and Chitwood, L.A. F ...
s, metal-rich
carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small prop ...
s,
mesosiderite
Mesosiderites are a class of stony–iron meteorites consisting of about equal parts of metallic nickel-iron and silicate. They are breccias with an irregular texture; silicates and metal occur often in lumps or pebbles as well as in fine-grained ...
s), and in some cases may also be rubble piles.
Formation of the M-type asteroids
The earliest interpretation of the M-type asteroids was that they were the remnant cores of early
protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationa ...
s, stripped of their overlying crust and mantles from massive collisions that are thought to have been frequent in the early history of the solar system.
It is acknowledged that some of the smaller M-type asteroids (<100 km) may have formed in this way, but that interpretation was challenged for
16 Psyche
16 Psyche () is a large M-type asteroid discovered by the Italian astronomer Annibale de Gasparis on 17 March 1852 and named after the Greek goddess Psyche.
The prefix "16" signifies that it was the sixteenth minor planet in order of dis ...
, the largest of the M-type asteroids.
There are three arguments against Psyche forming in this way.
First, it must have started as a Vesta-sized (~500 km) protoplanet; statistically, it is unlikely that Psyche was completely disrupted while Vesta remained intact. Second, there is little or no observational evidence for an
asteroid family
An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions. An a ...
associated with Psyche, and third, there is no spectroscopic evidence for the expected mantle fragments (i.e. olivine) that would have resulted from this event. Instead, it has been argued that Psyche is the remnant of a protoplanet that was shattered and gravitationally re-accumulated into a well-mixed iron-silicate object.
There are numerous examples of metal-silicate meteorites, aka
mesosiderite
Mesosiderites are a class of stony–iron meteorites consisting of about equal parts of metallic nickel-iron and silicate. They are breccias with an irregular texture; silicates and metal occur often in lumps or pebbles as well as in fine-grained ...
s, that might be objects from such a
parent body
In meteoritics, a parent body is the celestial body from which originates a meteorite or a class of meteorites.
Identification
The easiest way to correlate a meteorite with a parent body is when the parent body still exists. This is the case fo ...
.
One possible response to this second interpretation is that the M-type asteroids (including 16 Psyche) accumulated much closer to the Sun (1–2 au), were stripped of their thin crust/mantles while still molten (or partially so), and later dynamically moved into the current asteroid belt.
A third view is that the largest M-types, including 16 Psyche, may be differentiated bodies (like 1 Ceres and 4 Vesta) but, given the right mix of iron and volatiles (e.g. sulfur), these bodies may have experienced a type of iron volcanism, aka ferrovolcanism, while still cooling.
Notable M-type asteroids
In the
JPL Small Body Database, there are 980 asteroids classified under the
Tholen asteroid spectral classification system.
Of those, 38 are classified as M-type.
Another 10 were originally classified as X-type, but are now counted among the M-types because their optical albedos fall between 0.1 and 0.3.
Overall, the M-types make up approximately 5% of the asteroids classified under the Tholen taxonomy.
(16) Psyche
16 Psyche
16 Psyche () is a large M-type asteroid discovered by the Italian astronomer Annibale de Gasparis on 17 March 1852 and named after the Greek goddess Psyche.
The prefix "16" signifies that it was the sixteenth minor planet in order of dis ...
is the largest M-type asteroid with a mean diameter of 222 km, and has a relatively high mean
radar albedo of
suggesting it has a high metal content in the upper few meters of its surface.
The
Psyche spacecraft is slated to visit 16 Psyche, arriving in early 2026.
(21) Lutetia
21 Lutetia
)
, mp_category=Main belt
, mpc_name=(21) Lutetia
, orbit_ref =
, epoch=May 31, 2020 ( JD 2459000.5)
, semimajor=2.435 AU
, perihelion=2.037 AU
, aphelion=2.833 AU
, eccentricity=0.16339
, period=3.80 yr (1388.1 d)
, inclination=3.064Â ...
has a mean diameter of 100 km,
and was the first M-type asteroid to have been imaged by a spacecraft when the
Rosetta space probe
''Rosetta'' was a space probe built by the European Space Agency launched on 2 March 2004. Along with ''Philae'', its lander module, ''Rosetta'' performed a detailed study of comet 67P/Churyumov–Gerasimenko (67P). During its journey to the ...
visited it on 10 July 2010.
Its mean radar albedo of
is roughly twice that of the average
S-type or
C-type asteroid
C-type (carbonaceous) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...
, and suggests its
regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
contains an elevated amount of metal phases relative to other asteroid classes.
Analysis using data from the Rosetta spectrometer (VIRTIS) was consistent with estatitic or iron-rich carbonaceous chondritic materials.
(22) Kalliope
22 Kalliope
Kalliope ( ; minor planet designation: 22 Kalliope) is a large M-type asteroid from the asteroid belt discovered by J. R. Hind on 16 November 1852. It is named after Calliope, the Greek Muse of epic poetry. It is orbited by a small moon named Li ...
is the second largest M-type asteroid with a mean diameter of 150 km.
A single moon, named
Linus
Linus, a male given name, is the Latin form of the Greek name ''Linos''. It's a common given name in Sweden. The origin of the name is unknown although the name appears in antiquity both as a musician who taught Apollo and as a son of Apollo who di ...
, was discovered in 2001
and allows for an accurate mass estimate. Unlike most of the M-type asteroids, Kalliope's radar albedo is 0.15, similar to the S- and C-type asteroids,
and does not suggest an enrichment of metal in its regolith. It has been the target of high resolution adaptive optics imaging which has been used to provide a reliable size and shape, and a relatively high bulk density of 4.1 .
(216) Kleopatra
216 Kleopatra, with a mean diameter of 122 km, is the third largest M-type asteroid known after 16 Psyche and 22 Kalliope.
Radar delay-Doppler imaging, high-resolution telescopic images, and several stellar occultations show it to be a contact binary asteroid with a shape commonly referred to as a "dog-bone" or "dumbbell."
Radar observations from the Arecibo radar telescope indicate a very high radar albedo of
in the southern hemisphere, consistent with a metal-rich composition.
Kleopatra is also notable for the presence of two small moons, named Alexhelios and Cleoselena, which have allowed its mass and bulk density to be accurately computed.
See also
*
Asteroid spectral types
An asteroid spectral type is assigned to asteroids based on their emission spectrum, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition. For small bodies that are not internally differentiate ...
References
{{Small Solar System bodies
Asteroid spectral classes
*
*